Why are tractor-trailer rigs called "semis"?

Dear Straight Dope:

I was on a road trip with some friends the last couple days, and we got to wondering why people call tractor trailer rigs semi trucks. Wouldn't a semi truck be only sort of a truck? I would consider an eighteen wheeler a full fledged truck. Can you enlighten me?

— Adam Sund

Dear Adam:

The full name of that thing sucking your doors off on the interstates is "tractor semitrailer." The part in the back is called a semitrailer because only the back end sits on wheels (and thus "trails"); the front end is supported by the tractor. A full trailer is fully supported by its wheels (commonly on axles at front and back, in the case of heavy over-the-road vehicles) and is connected to the tractor (or, far more commonly, the semi in front of it) by a drawbar. A truck need not have a trailer to be considered a truck, of course. Look at one of the trucks you can rent from U Haul or Ryder. No trailers there.

— Jeff

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